while True:
    s=input("请输入0~255之间的字符串")
    if len(s)<=0 or len(s)>255:
        continue
    a=''
    for i in range (0,len(s)):
        if  s[i].isalpha():
            a=a+s[i]
    a=a[::-1]
    print('加密后的字符串为')
    for i in a:
        print(chr((ord(i)-ord('a')+3)%26+ord('a')),end='')
    break